1. Field of the Invention
This invention relates to an improved method for preparing stabilized alkyl glycosides by treatment with a metal borohydride after a peroxide bleaching step.
2. Description of the Related Art
Alkyl glycosides are conveniently prepared by reacting an alcohol of the type and chain length which is desired to form the "alkyl" portion of the glycoside of interest with a saccharide reactant (e.g., a monosaccharide such as glucose, xylose, arabinose, galactose, fructose, etc., or a polysaccharide such as starch, hemicellulose, lactose, maltose, melibiose, etc.) or with a glycoside starting material wherein the aglycone portion thereof is different from the alkyl substituent desired for the ultimate alkyl glycoside product of interest. Typically, such reaction is conducted at an elevated temperature and in the presence of an acid catalyst. Various alkyl glycoside products and processes for making same are disclosed in U.S. Pat. No. 2,235,783 (White, issued Mar. 18, 1941); U.S. Pat. No. 2,356,565 (Chwala, issued Aug. 22, 1944); U.S. Pat. No. 2,390,507 (Cantor, issued Dec. 11, 1945); U.S. Pat. No. 2,442,328 (Young, issued Jun. 17, 1947); U.S. Pat. No. 3,219,656 (Boettner, issued Nov. 23, 1965); U.S. Pat. No. 3,375,243 (Nevin et al., issued Mar. 26, 1968); U.S. Pat. No. 3,450,690 (Gibbons et al., issued Jun. 17, 1969); U.S. Pat. No. 3,547,828 (Mansfield et al., issued Dec. 15, 1970) U.S. Pat. No. 3,598,865 (Lew, issued Aug. 10, 1971); U.S. Pat. No. 3,640,998 (Mansfield et al., issued Feb. 8, 1972); U.S. Pat. No. 3,707,535 (Lew, issued Dec. 26, 1972); U.S. Pat. No. 3,721,633 (Ranauto, issued Mar. 20, 1973); U.S. Pat. No. 3,737,426 (Throckmorton et al., issued Jun. 5, 1973); U.S. Pat. No. 3,772,269 (Lew, issued Nov. 13, 1973); U.S. Pat. No. 3,839,318 (Mansfield, issued Oct. 1, 1974); U.S. Pat. No. 3,974,138 (Lew, issued Aug. 10, 1976); U.S. Pat. No. 4,011,389 (Langdon, issued Mar. 8, 1977); and U.S. Pat. No. 4,223,129 (Roth et al., issued Sep. 16, 1980).
In the preparation of alkyl glycoside products, it is not uncommon for such products to develop an undesirably dark coloration during the course of the synthesis and isolation procedures employed. Various procedures have been suggested for improving the color of such dark colored glycoside products including, for example, treatment with bleaching reagents such as hydrogen peroxide; intentional color formation by heat treatment under alkaline conditions followed by removal (e.g., by precipitation, filtration, etc.) of dark colored impurities generated during said treatment procedure; treatment with decolorizing adsorbents such as particulate carbon materials, etc.; and the like. See in this regard, for example, Gibbons' U.S. Pat. No. 3,450,690 which discloses an alkaline heat treatment/separation procedure that can optionally be followed by treatment with bleaching agents such as hydrogen peroxide or by treatment with decolorizing carbons. See also Cantor's U.S. Pat. No. 2,390,507; White's U.S. Pat. No. 2,235,783; Example 1 of Throckmorton et al.'s U.S. Pat. No. 3,737,426; Examples 5 and 10 of Langdon's U.S. Pat. No. 4,011,389; and Example 1 of U.S. Pat. No. 4,472,170 to Hellyer (issued Sep. 18, 1984) for teachings related to the use of carbon adsorbents for the decolorization of various alkyl glycoside products.
Even when glycoside products are originally prepared (or are subsequently decolorized in accordance with one or more of the procedures set forth above) in a fashion which results in initial color characteristics acceptable for certain applications, such products nonetheless commonly exhibit a propensity to discolor (i.e., darken) as a function of time even under relatively mild storage conditions (e.g., at neutral or slightly acidic pH and normal room temperatures, i.e., 20.degree.-25.degree. C.). The propensity to discolor is greatly accentuated (i.e., in terms of the intensity and rapidity thereof) by exposure to elevated temperatures (such as, for example, in the range of 40.degree. C. to 100.degree. C. or more) and/or exposure to relatively strong alkaline aqueous environments (i.e., pH of 8 to 12). Generally speaking, the extent of discoloration is related to the severity of the pH/temperature/time to which the glycoside product is exposed. In U.S. Pat. No. 4,557,729 to McDaniel et al. (issued Dec. 10, 1985), the aforementioned problem of color deterioration of glycoside products during storage thereof is discussed and a method for obviating such problem is disclosed which entails first bleaching the glycoside product of interest with an oxidizing agent such as ozone, hydrogen peroxide, hypochlorite, etc., and thereafter exposing the resulting bleached glycoside product to a source of sulfur dioxide (e.g., sulfur dioxide gas, sodium sulfite, sodium metabisulfite, sodium hydrosulfite, etc.) to stabilize said glycoside product against color degradation. While the indicated method has been found to be quite effective in stabilizing the color of glycoside products against deterioration or darkening thereof under relatively mild storage conditions (e.g., at pH's in the range of from about 3 to about 7 and at temperatures in the range of from about 20.degree. C. to about 30.degree. C.), it has also been found to be not nearly as effective (and, in fact, less effective than is desired in many cases) in stabilizing against color deterioration under harsher conditions such as those involving prolonged storage at elevated temperatures (e.g., 35.degree. C. to 60.degree. C. or more) and those involving relatively high pH (e.g., pH=8 or more) environments, even in situations involving relatively short term/low temperature exposure. Accordingly, it would be highly desirable to provide a method for imparting improved high temperature and/or alkaline color stability to glycoside products which are otherwise prone to darken substantially upon exposure to high temperatures and/or alkaline conditions. European patent application 0387913 teaches that a method of producing an alkyl glycoside which is stable in hue and in color which utilizes a metal borohydride decolorization step which comprises treating an aqueous solution of the alkyl glycoside having a pH of about 8.7 to 9.3 with hydrogen peroxide, eliminating the residual hydrogen peroxide through reaction with a metal borohydride and thereafter decomposing the borohydride with acid. European patent application 0388857 teaches a method of producing an alkyl glycoside which is stable in hue and in color which utilizes a metal borohydride decolorization step which comprises reacting a reaction mixture containing an alkyl glycoside and unreacted higher alcohol with metal borohydride. U.S. Pat. No. 4,959,468 teaches a method for improving the color stability of glycoside products which comprises treating the glycoside product by contacting the glycoside product with a color stabilizing amount of from about 0.01 to about 2 weight percent, on a glycoside product dry weight basis, of a borohydride material selected from the group consisting of Group I or Group II metal borohydride salts for a time period sufficient to substantially reduce the propensity of the glycoside product to darken upon exposure to elevated temperatures under alkaline conditions. This method does not eliminate haze which may develop in aqueous solutions of alkyl polyglycoside products having a pH in the 6-8 range. There are no methods in the prior art which improve the color stability of a glycoside product and eliminate haze. It is therefore, an object of the present invention to not only improve the color stability of a glycoside product but also to clarify the glycoside product by substantially eliminating haze which may be exhibited by aqueous solutions of alkyl polyglycoside products having a pH in the 6-8 range.